<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>display</title>
</head>

<body>
    <span class="circle">1</span>
    <span class="circle">2</span>
    <span class="circle">3</span>
    <span class="circle">4</span>
    <span class="circle">5</span>
    <span class="circle">6</span>
    <span class="circle">7</span>
    <div class="circle">8</div>
    <div class="circle">9</div>
    <div class="circle">10</div>
    <div class="circle">11</div>
    <div class="circle">12</div>
    <div class="circle">13</div>
    <div class="circle">14</div>
    <p class="circle">15</p>
    <p class="circle">16</p>
    <p class="circle">17</p>
    <p class="circle">18</p>
    <p class="circle">19</p>
    <p class="circle">20</p>
    <p class="circle">21</p>
    <style>
        body {
            width: 1000px;
            height: 500px;
            margin: auto;
        }

        .circle {
            display: inline-block;
            height: 50px;
            width: 50px;
            color: #39C5BB;
            border: 2px solid gray;
            border-radius: 50%;
            text-align: center;
            line-height: 50px;
            font-size: 25px;
            margin: 10px 10px 10px 10px;
        }

        .circle:nth-child(2n) {
            background-color: #66CCFF;
        }

        .circle:nth-child(2n+1) {
            background-color: brown;
        }
    </style>
</body>

</html>